This invention is directed to a so called double acting valve seat which is responsive to both flow line fluid pressure and valve body fluid pressure to effect sealing thereof. The double acting valve seat assembly of this invention is shown here in a ball valve and in a gate valve. Normally, double acting valve seats are constructed so that when the flow line fluid pressure is greater than the valve body pressure then a greater area at the rear portion of the seat assembly is exposed to the flow line fluid pressure so the entire seat assembly is urged toward the valve member. Likewise, when the body fluid pressure is greater than the flow line fluid pressure the seat assembly is so constructed that the body fluid pressure is exposed over a greater area of the rear portion of the seat assembly than on the face portion thereof so the seat assembly is urged toward the valve member by force generated because of the high body fluid pressure.
Typical prior art double acting valve seat assemblies employ a one piece construction with a pair of back face seals radially from each other and movable between two positions to provide a greater rear area which is exposed to the fluid pressure when either the flow line fluid pressure is greater than the valve body fluid pressure or when the valve body fluid pressure is greater than the flow line fluid pressure. For example, typical prior art double acting seat assemblies are shown in U.S. Pat. Nos. 3,421,733 and 3,834,664. In these prior art devices the entire seat member has to be displaced or urged toward the valve member when fluid pressure is greater on the back side of the seat. Because the entire seat assembly must be displaced a substantial fluid pressure differential is required to properly actuate the seat. For low pressure valve applications and applications where the nature of the lading is such that the seats might stick, then operation of the prior art valve seats may be impaired or prevented.